Previously, we demonstrated that the phorbol ester, 12-0-tetradecanoyl-phorbol-13-acetate (TPA) was effective in eliciting a dense outgrowth of neurites from 9 day old embryonic sensory ganglia. We will extend our survey of neuronal targets responsive to TPA and attempt to define the molecular mechanisms underlying the TPA effects. We suggest further studies which will compare the neurite outgrowth response of central neuronal tissues including explants from the spinal cord, cerebral cortex and cerebellum. The receptor binding characteristics of sensory neurons from chick embryos ranging from 9-20 days in age will also be studied. Our goal is to extend our previous observations of the neurite promoting effects of TPA so that we can evaluate its action in terms of neuronal target and age specificity. Since the Ca++/phospholipid-dependent protein kinase (protein kinase C, or PK-C) is most likely the cellular phorbol ester receptor within the cell, we will correlate the neurite-promoting effects of TPA with activation of PK-C. We will test if treatment of neuronal cultures with known activators of PK-C such as diacylglycerol and phospholipase C will reproduce the neurite-promoting effects of TPA. Inhibitors of PK-C will also be employed to determine if they are effective in preventing the outgrowth of neurites induced by TPA. To begin formulating the molecular basis of TPA action, we will search for native substrates proteins of PK-C in neuronal cells. Three other TPA effects which may bear relevance to neurite outgrowth will be studied. The induction decarboxylase and plasminogen activator by TPA will be correlated with neurite outgrowth. Changes in the surface glycoproteins of neuronal cells subsequent to TPA treatment will also be examined both morphologically and biochemically. These experiments should provide useful information on the molecular basis of neuritogenesis and the physiological significance of phorbol ester receptors, most likely PK-C, in neuronal development.